The 3rd Generation (3G) mobile communication system employs a Code Division Multiple Access (CDMA) mode and supports multimedia services. At present, the 3rd Generation Partnership Project (3GPP) has started up a Long Term Evolution (LTE) system using 3G radio interface technologies, so that the delay is reduced, the user data rate is increased, the system capacity and coverage is improved, and the cost of the operator is lowered.
Among the three international standards of the 3G mobile communication systems, the Time Division-Synchronized Code Division Multiple Access (TD-SCDMA) system employs the Time Division Duplex (TDD) mode. The TD-SCDMA system supports asymmetric uplink and downlink service transmission and has a relatively high flexibility on the utilization of frequency spectrum. The TD-SCDMA system synthetically employs the advanced technologies in wireless communications such as smart antenna, uplink synchronization, Joint Detection and software radio, so that the system has high performance and frequency spectrum utilization.
In FIG. 1A which shows a schematic diagram of a frame structure of a TD-SCDMA system, a radio frame of 10 ms contains two half-frames each of 5 ms, and each of the half-frames contains 10 time slots. In FIG. 1A, TS1-TS6 denote normal time slots (in the TD-SCDMA system, a normal time slot may also be referred to as a service time slot); TS0 denotes a downlink broadcast time slot; DwPTS refers to a downlink pilot time slot; GP refers to a guard period; and UpPTS refers to an uplink pilot time slot.
In a TD-SCDMA LTE solution, the following two types of frame structures are provided for an LTE system.
1) The first type of frame structure is applicable to Frequency Division Duplex (FDD) and TDD systems, as shown in FIG. 1B.
The length of the first type of radio frame is 10 ms, and the radio frame consists of 20 time slots which are labeled from 0 to 19 and each have a length of 0.5 ms, and two consecutive time slots are defined as a subframe.
For an FDD system, 10 subframes are available for the uplink and the downlink respectively in every 10 ms, because the uplink and the downlink are separated in the frequency domain; while for a TDD system, 10 subframes are shared by the uplink and the downlink in every 10 ms, and each subframe is allocated to either the uplink or the downlink. However, subframe 0 and subframe 5 are always used for downlink data transmission.
2) The second type of frame structure is applicable to a TDD system, as shown in FIG. 1C.
The length of the second type of radio frame is also 10 ms, and each radio frame is divided into 2 half-frames each of 5 ms, and each half-frame consists of 7 normal time slots (labeled from 0 to 6) and 3 special time slots (a DwPTS, a GP and an UpPTS). Each normal time slot is defined as a subframe. Subframe 0 and the DwPTS are always used for downlink data transmission, while the UpPTS and subframe 1 are always used for uplink data transmission.
In order to be compatible with a TD-SCDMA system, an LTE-TDD system employs the second type of frame structure.
For a TDD system, in order to avoid the interference between the uplink and downlink time slots, a GP is needed for a switch point from the downlink time slot to the uplink time slot.
In order to increase the transmission efficiency of the LTE-TDD system based on the second type of frame structure, enhance the flexibility on the configuration of the special time slots and support different coverage ranges flexibly, the current solution is as follows.
The length of a Cyclic Prefix (CP) of each Orthogonal Frequency Division Multiplexing (OFDM) symbol in the second type of frame structure of the existing LTE-TDD system is shortened, both of the long CP and the short CP should be shortened correspondingly, and specific frame structure parameters are as shown in Table 1. The total length of each normal time slot is 20736 Ts (0.675 ms), and the normal time slot consists of a data part with a length of 19744 Ts (0.6427 ms) and a Guard Interval (GI) part with a length of 992 Ts (32.29 μs) for a switch between time slots. In the case of a short CP configuration, the data part of each normal time slot consists of 9 OFDM symbols, the CP length of the first OFDM symbol is 160 Ts (5.21 μs), and the CP length of each of the other 8 OFDM symbols is 144 Ts (4.69 μs), which is consistent with the first type of frame structure. In the case of a long CP configuration, the data part of each normal time slot consists of 8 OFDM symbols, the CP length of each of the first and the second OFDM symbols is 432 Ts (14.06 μs), and the CP length of each of the other 6 OFDM symbols is 416 Ts (13.54 μs).
Here, Ts is a unit of sampling time, and 1 Ts=1/(15000*2048) seconds.
TABLE 1Parameters of LTE-TDD frame structure after adjusting the CP lengthLength ofNormalNumber ofTime SlotOFDM SymbolsCP LengthShortA total Length of9CP length of theCP length of each ofCP20736 Ts (0.675 ms),First OFDM Symbolthe other 8 OFDMwith a Data Part ofis 160 TsSymbols is 144 Ts19744 Ts (0.6427 ms)(5.21 μs)(4.69 μs)Longand a GI Part of8CP length of eachCP length of each ofCP992 Ts (32.29 μs)of the First 2 OFDMthe other 6 OFDMSymbols is 432 TsSymbols is 416 Ts(14.06 μs)(13.54 μs)
The redundant time length may be used for extending the special time slots by overlapping the GI part of each normal time slot on the data part of the adjacent time slot.
FIG. 2A shows a frame structure in which the special time slots are extended by overlapping the GI part of each normal time slot on the data part of the adjacent time slot when the GI part of each normal time slot lies in front of the data part of the normal time slot.
FIG. 2B shows a frame structure in which the special time slots are extended by overlapping the GI part of each normal time slot on the data part of the adjacent time slot when the GI part of each normal time slot lies behind the data part of the normal time slot.
In addition, the GI part in time slot TS0 may not overlap the data part of the adjacent time slot, as shown in FIGS. 2C and 2D.
After the GI part of each normal time slot overlaps the data part of the adjacent time slot, no GI part actually exists between the data parts of the time slots, and an equivalent frame structure thus formed is as shown in FIG. 2E, where the length of the data part of each normal time slot is 642.7 μs, the length of the special time slot is 501.04 μs, and the special time slot consists of three parts including a DwPTS, a GP and an UpPTS; the DwPTS is used for transmitting a primary synchronization signal, and the UpPTS is used for random access.
In the case that the total length of the special time slot keeps constant as 501.04 μs, the time slot lengths of the three time slots, i.e., the DwPTS, the GP and the UpPTS, may be adjusted flexibly, where the minimum length of the DwPTS is the length of an OFDM symbol that contains a long CP, i.e., 80.73 μs; when the UpPTS is used for random access, its minimum length is the length of two OFDM symbols that contain no CP, i.e., 133.33 μs. By adjusting the lengths of the three time slots in the special time slot, different coverage ranges may be supported flexibly, and the transmission efficiency may be increased.
Although, via the above LTE-TDD frame structure, the transmission efficiency of the LTE-TDD system can be increased effectively and different coverage ranges can be supported flexibly, because the time slots of the frame structure modified are not aligned with the time slots of the frame structure of the TD-SCDMA system, serious interference between the systems may be caused when the LTE-TDD system and the TD-SCDMA system are co-located in adjacent channel frequency. As shown in FIG. 2F, in the frame structures of the LTE-TDD system and the TD-SCDMA system, if time slot allocation ratios in the two systems are the same, that is, the uplink-to-downlink switch points are both configured between time slots TS3 and TS4, then a part of the uplink time slot TS3 of the LTE-TDD system falls in the downlink time slot TS4 of the TD-SCDMA system, which results in that a base station including the LTE-TDD system may directly receive the signals transmitted by a base station including the TD-SCDMA system, thereby causing serious interference between the systems.
In summary, at present, when the LTE-TDD system and the TD-SCDMA system are co-located in adjacent channel frequency, serious interference between the systems may be caused.